Sunday, June 28, 2009

S-PRISM Scalability a Repose for Steven Kirsch

Steve Kirsch has posted an important statement on the IFR today on The Huffington Post. While Kirsch's statement is something of a breakthrough for Nuclear Power on Huffington Post, It contains a major flaw that Kirsch was unaware of . The flaw is simple. IFR/S PRISM technology is not sufficiently scalable to make a difference in the fight against AGW. I spotted the problem in a paper on the S PRISM fuel cycle. I discussed the scalability problem in a posted a month ago. I am reposting "Scalability and Breeder Start Up." Because I believe that the backers of nuclear power should openly debate their options, I intend to publish more posts on the IFR/S PRISM option, and my questions about it in future Nuclear Green posts.

Scalability and Breeder Start Up
Scalability is a deal breaker in global warming technology. One of the nice things about the LFTR is that it is scalable. You can build them in factories ship them off to coal-fired generation facilities, dig a whole into the ground, plant them, hook um up to the Grid, and turn them on. And then stand back and let them work. Every now and then you might add some thorium and remove some U-233 that would be used to start a new reactor.

Basically you could build as many as you wanted too in the LFTR factory. You would need a start up charge of fissionable material - U-233, U-235, or Pu-239. The start up charge would initiate the chain reaction in the reactor, and begin the breeding process. Later fuel will be derived from breeding, so no further nuclear fuel from external sources would be required to keep the chain reaction going.

The number of start up charges, the material composition of start up charges, and the size of each charge would pose a potential limit on LFTR scalability. LMFBRs would also require start up charges.

Neutron speed would play an important role with faster neutron reactors requiring more fissionable materials to keep a chain reaction going. For example French researchers studying Molten Salt Reactors operating at various neutron speeds found that a Thermal TMSR requited a charge of 790 kgs of U-233 in order to maintain breeding in a 1 GWe reactor. An Epithermal TMSR required 2400 kgs to fulfill the same conditions. While a Fast TMSR required 5200 kgs of U-233. The French also reported that a standard fast neutron reactor - I assume a LMFBR -would require 12,25o kgs of plutonium.

An S PRISM related study "S-PRISM Fuel Cycle Study: Future Deployment Programs and Issues," suggested that as of the year 2000, four hundred tons of plutonium could be recovered from spent nuclear fuel. This in turn would provide enough plutonium to supply start up charges for twenty-two, 1520 MWe S-PRISM facilities with ab output of 33,440 MWe. That is about 12 tons per 1 GWe of reactor capacity.

Clearly then neutron speed has an adverse effect on reactor scalability.

On the other hand neutron speed also influences the fission rate per neutron absorption, this in turn influences neutron production. Pu-239 fissions 25% more often in a fast reactor than in a thermal reactor. On the other hand it still take more Pu-239 to maintain a chain reaction in a fast reactor than in a thermal reactor. Reactor physics tricks and fuel cycle also seem to influence start up charge size.

A recent discussion on the EfT form produced quite a lot of useful information. "Jagdish" reported that
Indian 500MW PFBR is designed to use only two tons of plutonium.

"Honzik" pointed to French research of epithermal/fast Thorium Molten Saalt Reactors. The French, modeling the use of transuranium materials from spent nuclear fuel, in a 1 GB reactor had calculated a need for 7.3 tons of fissile elements (87.5% of Pu (238Pu 2.7%, 239Pu 45.9% , 240Pu 21.5%, 241Pu 10.7%, and 242Pu 6.7%), 6.3% of Np, 5.3% of Am and 0.9% of Cm). Alternatively the reactior would require a start uo charge of 4.6 tons of U-2330.

Lars reported that
The minimum for unity breeding from the French group is 1.5 tonnes u233 / GWe.
Alex P noted:
the french design has an only radial, not axial, blanket, so for comparison I'd think that the fissile start-up in a LFTR with a fully encompassing blanket can be at least one tonn of u-233 per GWe, or even lower
David LeBlanc noted:
The French TMSR design running without graphite moderator needs upwards of 5 tonnes of U233 or 8 or more tonnes of fissile Pu. They could drop this somewhat if they just wanted to barely break even but not very much since they'll start losing too many neutrons that would migrate into the axial reflectors. In designs in which the blanket is nearly fully encompassing you can get by with much lower fissile concentrations. It is only speculation for now but based on early Oak Ridge studies using sphere within sphere designs I think we could probably get things down to 500 Kg of u233 or maybe even lower but 1000 kg is a fine for a conservative estimate. These designs with lower fissile concentration would also be fairly soft spectrums since the salt itself can do a modest job at moderating the neutrons.
The problem of plutonium in nuclear breeding should be noted. In thermal breeders plutonium suffers from poor neutron economy, while in fast neutron reactors plutonium neutron economy improves but does not compensate for the added requirement for fissile material. Radial and axial thorium blankets in a breeder appears to lower fissile demand by as much as 300% (but this principle has been applied in S-PRISIM design).

The S-PRISM design would appear far less scalable than Epithermal or thermal MSRs. David LeBlanc's estimates are based on the use of blankets with Epithermal MSRs. If we estimate that 2 kgs of reactor grade plutonium from spent nuclear fuel about 1 kg of U233, 500 kgs of U-233 would be a similar startup charge to a ton of RGP. Thus the same amount of RGP that will start 33 GWe worth of S-Prism FBRs will also start 400 GWe worth of LFTRs. Clearly the LFTR offers scalability advantages over the IFR/S-PRISM.

13 comments:

Barry Brook said...

ALMRs, configured for maximum blanket breeding, can achieve a breeding ratio of 1.25:

http://books.google.com.au/books?id=iRI7Cx2D4e4C&pg=PA55&lpg=PA55&dq

That's a doubling time of 72/25 = 2.88 years. What does this mean?

Take a 30 year period (2020 to 2050) with an initial 22 x 1.5 GWe of IFRs (there would be more start charges than this available, given the ongoing output of Gen II/III between 2000 and 2020 and potential use of military plutonium).

Anyway, this initial fleet can breed sufficient start charges for an additional 1.25^30 = 807 x 33.4 = 26,954 GWe of capacity by 2050. Give it another 10 years, and you have enough for 251,274 GWe. You'd not be needing to breed much above a few percent, if at all, after this point!

Further, given the parallel builds of a large swag of Gen III reactors over the 2010 to 2030 period (this is inevitable, especially in places like China), the reality of start charge availability is even more optimistic than this.

In short, it's a non issue, whether your flavour is MSR or IFR.

Charles Barton said...

Thank you Berry. I was looking at a Breeding ratios of 1.22 and 1.05. Even 1.25 is easy to acompllish after with plutonium. However, nothing beats thermal breeders for rapid deployment. By now you can get 600 GWs online just as fast as they flow off the assembly libe, get transporteds, and hooked up to the grid.

Soylent said...

"That's a doubling time of 72/25 = 2.88 years. What does this mean?"

I think you've made a mistake here.

A breeding ratio of 1.25 is unitless, it's fuel produced versus fuel fissioned. If you knew the time it takes to fission the equivalent of one start-up charge worth of fissile material and you multiply that with ln 2/ln 1.25 ~ 72/25 you get the doubling time.

This doubling time is still a bit iffy. It assumes that fuel is reprocessed almost straight out of the reactor, it takes very little time to extract the actinides, it takes very little time to fabricate new fuel elements and there is always a completed reactor waiting for it's start-up charge somewhere in the world. I profess ignorance on how close these assumptions are to being true, but it is something that needs to be examined.

Soylent said...

I figure the doubling time is somewhere around 20 to 30 years for a reactor with a breeding ratio of 1.25, a start-up charge of 12 tonnes and a thermal output of 3 GW(~1 GWe). Please point out any errors you find in the following reasoning:

Each fission produces ~190 MeV usable energy(4% is carried away by anti electron-neutrinos that just barrel straight through the Earth and escape the solar system). That's ~18 TJ per mole. A mole of Pu-239 weighs ~239 grams. 12 tonnes of plutonium is 50 kilomoles. At 3 GW thermal you're consuming ~0.17 millimole per second, that's gives about 9 years to consume the equivalent of a start-up charge giving a 3 tonne plutonium surplus.

Since you can pool the surplus from multiple reactors and use that to start up a new reactor instead of waiting on the output from one reactor you can shorten the doubling time from 4*9 years to 72/25 * 9 years ~ 26 years.

Kirk Sorensen said...

Alvin Weinberg and others at ORNL in the 1960s took a dim view of the breeding claims of the LMFBR crowd. Here's a quote from one of their papers:

"The curves in Figure 2 indicate that the breeding gain and doubling time in themselves are not adequate measures of the ability of a breeder reactor to limit the amount of uranium ore that must be mined to fuel a large nuclear power economy. The fissile inventory is also important and a low specific inventory is particularly important in a rapidly growing nuclear economy. It is their low specific inventory that makes it possible for molten-salt thermal breeder reactors to compete with fast breeder reactors in limiting the resource requirements."

Barry Brook said...

Soylent, I don't understand what you are saying in your 2nd post. A breeding ratio of 1.25 simply means that you are breeding 25% more fissile material than you are consuming, per unit time. Yes, there are some delays in reprocessing, shipping charges etc., but it would be minor -- with IFRs using pyroprocessing, you'd be processing the breeder blanket and metal fuel rods on a regular basis.

Kirk & Charles, I make no argument against he proposition that thermal/epithermal MSRs will be efficient breeders with lower start charge requirements, and think it likely that they will compete strongly with LMFBRs. I support the dual RD&D of IFRs and LFTRs -- both are hugely promising. Just making the point that a current limitation on the supply of start charges are not a strong argument against the deployment of IFRs. The low specific inventory referred to by Weinberg in the 1960s is not such a concern 40+ years later, given all the Pu now in existence in LWR spent fuel.

Charles Barton said...

Barry, you have to understands that the Molten Salt Reactor wasd developed in Oak Ridge presicely because Oak Ridge scientists and engineers thought that sodium cooled reactors were dangerous, and despite having developed an extremely safe and highly effective reactor, they saw federal funding go to the LMFBR, and their own. Indeed had only a quarter of the funds that were wasted on LMFBRs been spent on MSR R&D, we would today have LFTRs producing power. The LMFBR/IFRs have been huge money pits, an enormous diversion from reactors whose coolants will never burn simply because they come in contact with air or water.

I find among IFR backers the dogma, that the IFR is perfect, that it is the ultimately safe reactor, and therefore we should not ask questions about IFR safety. Yet when I reviewed IFR working documents from 1994, I noted significant safety concerns among IFR developers. Thus I view you, Steve, and Tom as engaging in hype about IFR safety, but you appear to have never looked at the original working documents and ask if the IFR is so safe, why were the developers worried?

You guys go on endlessly about the safety of the IFR as if it will never brake, when they are in fact one huge potential disaster waiting to happen. Finally you are an Aussie. It is none of your business which nuclear technology the United States Government chooses. We are Americans. It is our business, because it is our taxes that pays for them.

Soylent said...

Barry; no, not per unit time. Time is not involved at all in the definition of breeding ratio.

Breeding ratio is the number of new fissile atoms created on average for each atom that undergoes fission.

Time only gets involved when you give the rate at which these fission reactions occur(given indirectly by thermal power output, which is limited to what you can safely cool and the amount of power you want to produce) and the size of the start-up charge you need.

To get a new start-up charge of 12 tonnes with a 1.25 breeding ratio you must fission 48 tonnes of plutonium to get a 12 tonne surplus.

Barry Brook said...

Righto Charles, if you think my non-US citizenship disqualifies me from discussing nuclear technology on your site, I'll acquiesce to your wishes and henceforth cease and desist from commenting here. It's your blog and I have no wish to intrude where I, or my opinions, are not wanted.

See you guys someplace else.

Charles Barton said...

Barry you are free to discuss technology on my blog. I am objecting to your post titled "Why is the US ignoring the Integral Fast Reactor?" Now I realize that you are using Kirsch's material, but I feel that you are stepping across the line between technology and politics, and in particular you were posting material intended to influence United States Government decisions. Kirsch is beating the tom toms for Further UNited States Government subsidies for IFR development. After a US government investment of over $20 billion in LMFBRS, and the glowing reports that you, Tom, and Steve have given for IFR technology, if it is not ready for private companies to take a risk on, why should we think that spendiong another $3 billion of the money of American tax payers money would improve the IFR's prospects?

As long as you discuss technology, I might disagree, but I am not going to object, but when you post material designed to advise the United States Government on how to spend my tax dollars, you have gone to far.

Anonymous said...

Charles the fact that your argument is that because Barry isn't an American as the basis for your outrage is not only disappointing but also why meaningful discussions about the pros and cons of such promising technology are so difficult. Why are you trying to build barriers where clearly bridges are needed

mark4asp said...

U-236 is 0.4% of SNF; that's 20% of the available fissionables from SNF.

U-236 fissions in a fast reactor but not in the thermal range.

You are only extracting Pu-239 from SNF. You can get just over twice as much fissionable material out by taking:
U-235, U-236, Pu-239, Pu-240
0.8%, 0.4% , 1.0%
[% are typical for LWR SNF]

Note: even though the Pu-240 may not be fissionable, we want to burn it up.

Consequently, double the number of IFRs.

mark4asp said...

George Stanford said that:

An IFR core has about 20 MTHM per GWe, and another 20 MTHM, or so, in ex-core inventory. This is enriched at 20% using fissionable material.

It requires 8 tonnes of fissionable material per GWe of reactor capacity.

Assuming are getting 2.2% of fissionables from the SNF [0.8% U-235, 0.4% U-236, 1% Pu]

I assume: 880 MTHM fissionables instead of 400 tonne plutonium.

I calculate enough to startup 110GWe of IFRs

See: Is a Traveling Wave reactor better than an IFR?

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